The present invention relates generally to driver circuits for light emitting diodes (referred to as LED hereinafter). More particularly, the present invention relates to LED driver circuits having control circuitry for protecting an LED from over-current damage resulting from a short circuit fault.
LEDs have in recent years become extensively developed and mass produced in accordance with advances in higher luminance. Typically, an LED is provided with an LED chip which emits blue light, a color conversion module including translucent sealing materials (e.g. such as epoxy resin and silicone resin), and a phosphor layer by which the LED chip is covered and light serving as a complementary color is emitted through absorption of light emitted from the LED chip.
LEDs can serve even more diversified purposes with the emergence of LEDs which are capable of emitting white light (i.e. so-called white LEDs). LEDs have advantages such as longer life, excellent responsiveness, and a compact configuration in comparison with incandescent lamps. Because LEDs can themselves be provided in a small and light-weight configuration, this enables the formation of a thin and three-dimensional lighting fixture as a whole, so there are further advantages realized such as enhancing a degree of freedom in the design of lighting fixtures. Furthermore, light with a desired color can be easily obtained without using color filters.
With the development of LEDs which are capable of replacing incandescent lamps and fluorescent lamps and realize light emission with high luminance in a high output, there have been further developments in the field of vehicle lighting by including not only interior lamps and daytime running lamps for vehicle visibility improvement in daytime, but also in headlights.
LED luminance is generally decreased by a temperature rise resulting from self-heating by lighting. Therefore, LED driver circuits have been proposed which maintain constant luminance by controlling a current flowing into the LED in accordance with a lighting time of the LED.
Such a driver circuit to light the LED is, for example, shown in FIG. 18. A DC/DC converter 2 is adapted to control a current supplied from a DC power source 1 to an LED 4. An output from the DC/DC converter 2 is subjected to a feedback control so that a current I2 detected by a resistor R1, which is connected in series with the LED 4 and provided for current detection, is brought into a substantially constant state.
However, the feedback control of this configuration is performed for any output from the DC/DC converter 2, so as to increase the amount of a current flowing into the LED 4 not only in the case of compensation for a voltage decrease resulting from self-heating of the LED 4, but also in the case of dealing with a decrease of a forward direction voltage Vf of the LED 4 due to a short circuit of the LED 4 or other such reasons. Therefore, if the LED 4 fails and short-circuited for whatever reason, a larger amount of a current will continuously flow into the LED 4.
The LED 4 is typically provided with an LED chip using compound semiconductor materials (e.g. InGaN-based and AlInGaP-based materials) fixed on a substrate by adhesives such as solder and resin. Electrical connection between the LED chip and a circuit pattern of an external substrate circuit is established where a conductor pattern arranged on the substrate and an electrode of the LED chip are electrically connected to each other by an electrical connection member such as for example solder and/or a metal wire. Furthermore, the LED chip, the electrical connection member, the substrate, or other components are electrically and mechanically protected by covering them with a cover made of, for example, resin or glass directly or with air or the like interposed there-between. A illumination device having such an LED 4 as a light source is configured so that the LED 4 is arranged and fixed inside a lamp and fixture chassis covered with translucent glass, plastic, and the like in the front.
The high output LED 4 uses a plurality of LED chips each having a chip size of several hundreds μm square (e.g. 300 μm square) in serial connection or uses an LED chip with a large area equal to or more than 1 mm square, wherein higher output is achieved with a current ranging from several hundreds of mA to several amps flowing therein.
Therefore, a short circuit occurring in the high output LED 4 will result in a large heating value with a corresponding sharp temperature rise, followed by not only extinguishing of the LED 4 but other problems. A short circuit of the LED chip causes local overheating due to current concentration or other reasons, and the heat causes thermal expansion of air included in the LED 4 and/or sharp thermal expansion by evaporation of water included in a resin which may be used for an adhesive, cover or like purposes, whereby the LED 4 may be possibly damaged. In such cases, a short circuit of the LED 4 is accompanied by the failing of one or more components which constitute the LED 4 (such as an LED chip, adhesive, a substrate and a cover) on a lamp and fixture of a illumination device, internally contaminating and/or damaging the lamp and fixture, and possibly hindering the reuse of the illumination device.
Particularly because the high output LED 4 often uses solder in place of a metal wire as an electrical connection material, the LED 4 tends to be damaged by a short circuit rather than an open circuit resulting from disconnection of a metal wire. Moreover, a short circuit of the LED 4 not only causes a complete short circuit but also complicates a magnitude of a decrease of the forward direction voltage Vf.
FIG. 19 shows the current I2 which flows into the LED 4 when the DC/DC converter 2 is subjected to the feedback control so as to realize constant luminance of the LED 4, in relation to the voltage V2 across the LED 4 (i.e. current/voltage characteristics of the LED 4) in the driver circuit of FIG. 18, exemplifying a current/voltage locus observed when the LED 4 is short-circuited. Note that FIG. 19 shows a normal lighting state of the LED 4 when the voltage V2 across the LED 4 is higher than a voltage Vst.
Usually, lighting of the LED 4 is followed by a gradual decrease of the forward direction voltage Vf associated with a temperature rise resulting from self heating of the LED 4, and luminance of the LED 4 is accordingly decreased. In compensation for the luminance decrease of the LED 4, a feedback control is performed to increase the current I2 supplied to the LED 4 and to further increase the luminance of the LED 4, wherein current/voltage characteristics are observed as shown by a solid line in FIG. 19. When the LED 4 is short-circuited, the current I2 which continuously flows along with a decrease of the forward direction voltage Vf of the LED 4 finally reaches zero as shown by a broken line in FIG. 19. As a result, a short circuit of the LED 4 is accompanied by a longer period of time for an over-current to flow into the LED 4 with an increased heating value of the LED 4, whereby damage to the LED 4 may result and possibly cause a large scale contamination, impairment, or other such problems in a lighting fixture.
It is also possible to consider disabling the above driver circuit by using a fuse or other means. However, this is based on the assumption that current is stopped after the LED 4 is short-circuited with a temperature rise in an LED chip or other components, which means protection of the driver circuit can be achieved without sufficiently preventing contamination and impairment in the lamp and fixture or other components due to the LED 4 being damaged.
There has been proposed a power conversion device capable of obtaining desired output power from an input power, wherein a switching power source using a switching element for power conversion and a constant voltage control adjustment is employed to protect the switching power source from an over-current and overpower condition. An output characteristic of such a power conversion device shows a chevron-shaped characteristic as shown in FIG. 20, wherein an output voltage Vout is usually constant and a current is decreased when an output current Iout exceeds a predetermined value (i.e. over-current).
With an object of preventing the LED 4 from being damaged by applying such a switching power source to an LED driver circuit, it has further been proposed to use a current control circuit for a constant current driving control because the LED 4 does not usually require a high voltage, and luminance control by a current is easier to carry out. Therefore, an output characteristic of the current control circuit using a constant current driving control is exemplified as shown in FIG. 21, wherein an output current Iout is usually constant and is decreased when an output voltage Vout reaches or exceeds a predetermined value. However, if the LED 4 has a short circuit failure (graphically labeled as SC), the constant current control causes a voltage decrease, leaving a problem that a current flowing into the LED 4 cannot be reduced even if the voltage V2 across the LED 4 is decreased due to a short circuit of the LED 4.